1. Field of the Invention
The present invention relates to a ball mill for preparing fine metal powder. The present invention also relates to a method for preparing fine metal powder.
2. Prior Art
Various types of ball mills have been proposed to improve the efficiency for preparing fine metal powder per unit time. Theoretically, the more power is supplied to a ball mill in order to prepare fine metal powder, the larger amount of fine metal powder can be produced. However, when power is fed into a milling chamber of the ball mill, only a very small part of the fed power is used for milling metal powder, and the rest is converted into heat in the milling chamber during the ball mill operation, so that the temperature in the milling chamber is elevated. The elevated temperature in the milling chamber causes such a problem that obtained metal powder is oxidized, ignited or aggregated. Therefore, it is required to remove the heat generated in the milling chamber during the ball mill operation.
If all the power consumed by an agitator provided in the ball mill during the ball mill operation is converted into a quantity of heat (Q.sub.0), the efficiency of the ball mill operation can be estimated by the energy density Q.sub.0 /V, where V is the inner volume of the milling chamber. It is demanded that a ball mill is arranged to operate continuously in the condition of a high energy density Q.sub.0 /V. However, the energy density Q.sub.0 /V of a conventional ball mill is calculated at 0.03 kW/l at most. If more power is fed to raise the energy density Q.sub.0 /V, it will be impossible to keep a heat balance in the milling chamber, and the milling chamber will overheat.
One way to solve the above problem is to seal the milling chamber with non-oxidative gas. This way helps prevent metal powder from oxidizing or igniting, but makes it difficult to efficiently discharge the heat in the milling chamber from the milling chamber because the milling chamber is sealed. However long the milling operation may be continued in this condition, fine metal powder cannot be prepared since milled metal powder is aggregated because of the elevated temperature in the milling chamber and so on.
This way also makes it impossible to extract or take out the milled metal powder until the ball mill operation is finished, resulting in that the obtained milled metal powder has various wide ranges of particle size. There is also existing coarse metal powder which remains not to be sufficiently milled in the milling chamber. Therefore, this way needs to separate the obtained metal powder into fine and coarse metal powders, and is impossible to mill the starting metal powder in the fine metal powder with the yield of 100%.
Japanese Patent Publication No. 33783 of 1978 discloses a ball mill having a milling chamber with a set of milling blades disposed therein. As the milling blades are rotated, it generates a gas stream to efficiently discharge the heat generated in the milling chamber. The gas stream also entrains the milled metal powder out of the milling chamber to a separator. The separator separates the metal powder into fine and coarse metal powders, and then the fine metal powder is extracted and the coarse metal powder is re-fed into the milling chamber. This conventional ball mill is characterized by that the generated heat can be removed by the gas stream caused by the milling blades. However, when a great deal of metal powder is fed into the milling chamber, a resistance occurs to decrease the rotation rate of the milling blades, so that the heat tends to stay in the milling chamber. Therefore, it is necessary that the ball mill operation is interrupted by temporal suspension for cooling the milling chamber. If not, the milling chamber will overheat.
Therefore, it is an object of the invention to provide an improved and unique ball mill to overcome the above problems.
Another object is to improve the efficiency for preparing fine metal powder by efficiently discharging heat generated in the milling chamber during the ball mill operation, so that the ball mill is able to operate without interruption in the condition of high energy density, especially Q.sub.0 /V.gtoreq.0.05.
Another object is to provide a ball mill so arranged that milled metal powder can be extracted continuously to prepare fine metal powder having a narrow range of particle size.